Semiconductor memory array circuits, and specifically, electrically erasable programmable read only memories (EEPROMs), are rapidly growing in popularity today. They are extensively used in modern digital and computer systems for storing data and instructional codes that manipulate data to perform the desired functions.
Referring to FIG. 1, a block and schematic diagram of a prior art EEPROM memory array circuit 10 is shown with associated circuitry for addressing and performing memory operations on the memory array circuit. The EEPROM memory array circuit 10 comprises a plurality of memory cells arranged in an array orientation, including rows and columns of memory cells. Each memory cell consists of a field effect transistor (FET) having a drain (D), a source (S), and a gate (G), as it is conventionally known. The gate of an EEPROM memory cell FET typically consists of a floating gate (FG) and a control gate (CG). Some prior art EEPROM memory circuits further include an erase gate (EG) in addition to the floating and control gates (not shown in the prior art memory circuit of FIG. 1). As it is well known, the floating gate typically is the portion of the memory cell that holds the data content in the form of stored electron charges. The control gate is used for addressing the memory cell in order to perform memory operations on the cell, including writing, erasing and reading operations.
As explained earlier, the memory cell FETs of the prior art EEPROM memory array circuit 10 are arranged in an array orientation, which includes rows and columns of memory cell FETs. The memory cell FETs within a row typically have their respective control gates (CG) connected in common by a conductive line, typically designated as the wordline. The memory cell FETs within a column typically have their respective drains (D) connected in common by another conductive line, typically designated as the bitline. Usually, the source (S) of each memory cell FETs are all connected in common, and is designated as the common source. Each memory cell typically includes its own individualized floating gate (FG). Although as drawn in FIG. 1 the columns of the prior art EEPROM memory array 10 are shown to be horizontal, and the rows are shown to be vertical, the memory circuit 10 was drawn this way to schematically illustrate the structural orientation of the memory array, as will be explained in more detail later.
As it is conventionally known, the bitlines of the prior art EEPROM memory array 10, designated herein as BL1 sequentially through BLm, including a BLi which designates the bitline corresponding to the i-th column of memory cell FETs, are typically coupled to the outputs of a column address decoder/Y-Mux 14. The wordlines, designated herein as WL1 sequentially through WLn, including a WLj which designates the wordline corresponding to the j-th row of memory cell FETs, are coupled to the outputs of a row address decoder 12. As it is conventionally known, the row and column address decoders 12 and 14 are employed for addressing a particular row and column of memory cell FETs that are both common to a selected memory cell FET for which a memory operation is to be performed on, such as a writing, erasing or reading operation. Also included is a memory operation circuit 16 used for performing memory operations on the memory cells of the array. The memory operation circuit 16 includes a sense amplifier 18 and output and input buffers 20 and 22, as it is conventionally known.
Referring to FIG. 2, a structural and schematic diagram of a portion of the prior art EEPROM memory array 10 is shown. The portion shown includes memory cell FETs that are in rows common to wordlines WL(j-2) sequentially through WL(j+3), and memory cell FETs that are in column common to bitlines BL(i-2) sequentially through BL(i+3). Structurally, the memory cell FETs of the prior art EEPROM memory array 10 are formed within and on a silicon substrate 24. The sources (S) of the memory cell FETs are formed as diffusion lines within the substrate 24, and run parallel to the wordlines to connect in common the sources of memory cells in each of the rows. That is, the memory cell FETs that are within a row of memory cells have their sources connected in common by the source diffusion line, such as source diffusion lines SL(K-1), SL(K) and SL(K+1) shown in FIG. 2. In addition, pairs of adjacent memory cell FETs along the bitline direction have common sources connected to their corresponding source lines. For example, as shown in FIG. 2, the memory cell FETs common to wordlines WL(j) and WL(j+1) have their sources connected in common by source diffusion line SL(k).
The floating gates (FGs) of the memory cell FETs of the prior art EEPROM memory array 10 are typically formed as islands of polycrystalline silicon formed over a portion over the FETs' channel in a split-gate type EEPROM, which herein is serving as an example, adjacent to respective diffused source lines. Each memory cell FET includes its own floating gate (FG) and is separated from the substrate by a thin oxide layer, as it is conventionally known.
The wordlines (WL1-WLn) of the prior art EEPROM memory cell FETs are formed as polycrystalline silicon conductive lines situated over the substrate 24. Portions of each polycrystalline wordline are situated over respective memory cell FETs which form the control gates of FETs in a respective row of memory cells. In the split-gate configuration, a portion of the polycrystalline silicon control gates are formed over the portion of a respective channel that is not underlying a respective floating gate (FG), and is separated from the substrate by a thin oxide, as it is conventionally known. Another portion of the polycrystalline control gate (CG) is formed over a respective floating gate (FG) and is separated therefrom also by a thin oxide layer, as it is conventionally known.
The bitlines (BL1-BLm) of the prior art EEPROM memory array 10 are formed as metal lines deposited over the substrate 24 and over the other elements of the memory cell FETs, and are separated therefrom by an insulating oxide layer. The polycrystalline bitlines connect in common the drains (D) of memory cell FETs in respective columns, where these drains are formed as diffused region within the substrate 24. In order to electrically connect the bitlines to the drains of the memory cell FETs, a contact needs to be made through an insulating layer and down to the diffused drain region of each memory cell FET. As it will be explained in more detail below, it is the size requirement of these contacts and the areas around the respective contacts that is an impediment to the further shrinking and densifying of the prior art EEPROM memory array circuit 10.
Referring to FIG. 3, a plan view of a portion of the prior art EEPROM memory array 10 is shown. Specifically, four memory cell FETs are shown having common wordlines WL(j) and WL(+1), and common bitlines BL(i) and BL(i+1). As discussed above, each memory cell FET includes a control gate (CG) formed as a portion of the polycrystalline silicon wordline, and floating gate (FG) formed as an island of polycrystalline silicon. Adjacent pairs of memory cell FETs along the bitline direction have their respective sources connected in common by the diffused source line SL(k). As it is conventionally known, memory cell FETs are separated from adjacent memory cell FETs along the wordline direction by a field oxide layer (FOX). Notably though, each memory cell FET includes diffused drain regions connected to respective bitlines by way of a drain contact.
Notice that the drain contact of each memory cell FET in the prior art EEPROM memory array 10 occupies a relative large area of the memory cell FET. This is because the drain contact region not only occupies the surface area of the drain contact, it also occupies an area between the drain contact and adjacent wordlines for the purpose of separating and insulating the drain contact from the conductive wordlines. If L1 is defined as the length of the drain contact and L2 is defined as the length of each space separating the drain contact from adjacent wordlines, then the length of the drain contact region is L1+(2.times.L2). Assuming a 0.45 micron design rule, typical values for L1 and L2 are about 0.45 and 0.3 microns, respectively. This results in a drain region having a length of 1.05 microns for each memory cell FET.
Because the trend today is to increase the memory array capacity of EEPROMs, more memory cells including their respective drain contact regions are produced within a given integrated circuit die size. When the number of drain contact regions is multiplied by the number of memory cell FETs in a given EEPROM memory array, the cumulative area of the drain contact regions occupy a substantial area of the memory array size.
Besides the larger cell size, the prior art device as shown in FIG. 3 also requires the floating gates to be fabricated as individual rectangles before the control gates are formed. With the trend of continuous scaling toward smaller devices, it becomes increasingly more difficult to minimize the corner rounding effects in making the rectangular floating gates during the lithography and etching process. The drain and source contact problem and the comer rounding effect problem both pose a severe limitation to the continuous reduction in scaling of such devices.
Therefore, there is a need to reduce or eliminate the drain contact region of memory cell FETs in order to decrease the overall size of an EEPROM memory array, or conversely, to increase the number of memory cell FETs per a given integrated circuit die size. Solutions to such needs would translate into cost savings for the EEPROM memory arrays.